1. Field of the Invention
The present invention relates to an organic binder for a mold, which is used for production of a phenol-urethane-based gas hardening mold or self-curing mold used in sand casting, a molding sand composition and a mold obtained by using the same.
2. Discussion of Related Art
Conventionally, as one of typical organic molds used in sand casting, there has been known a phenol-urethane-based mold produced by using, as binder components, a polyol compound consisting of a phenolic resin and a polyisocyanate compound, and utilizing a polyaddition reaction (urethanization reaction) therebetween. As this phenol-urethane-based mold, there have been widely known a mass production type gas hardening mold produced by an amine cold box process, which requires no heating in production thereof, and a non-mass production type self-curing mold produced by an ordinary temperature self-curing process.
Specifically described, in the above-mentioned amine cold box process, the gas hardening mold is generally produced by kneading a granular refractory molding sand with an organic binder for a mold, which is composed of an organic solvent solution of a phenolic resin and a polyisocyanate compound solution, by using a mixer, for thereby producing a molding sand composition in which the surface of the molding sand is coated with the organic binder, and thereafter blowing the molding sand composition into a suitable forming mold to form a mold, and passing an amine-based catalyst gas through the forming mold to cure the molding sand composition. On the other hand, in the ordinary temperature self-curing process, the self-curing mold is produced by mixing a curing catalyst with a granular refractory molding sand and an organic binder for a mold, which is composed of an organic solvent solution of a phenolic resin and a polyisocyanate compound solution, during kneading of the molding sand and the organic binder, and immediately forming the resulting mixture into a desired shape.
As described above, in the amine cold box process, the molding sand composition is cured by bringing the amine-based catalyst gas into contact with the molding sand composition. In such a molding sand composition, however, the urethanization reaction gradually proceeds at a stage where the phenolic resin component and the polyisocyanate component that constitute the organic binder are mixed with each other, giving rise to a risk that the molding sand composition begins to cure at this stage. Accordingly, in the case where the mold is produced by using the molding sand composition stored in a sand hopper in a production setting for a long period of time, particularly in the case where the mold is produced in a high temperature and humidity environment, for example, the produced mold has a strength significantly lower than the desired mold strength, and sometimes various production failures are caused. Thus, production troubles due to the pot life of the molding sand composition tend to be caused. Further, where the pot life of the molding sand composition is short, there arises a need for additional work for removing the molding sand composition that is left in the sand hopper and became unusable, leading to reduction of the production efficiency of the mold.
Under the above-described circumstances, various compounds have been proposed as pot life extenders or curing retardants for extending the pot life of the molding sand composition. For example, U.S. Pat. No. 4,436,881 (Patent Document 1) proposes organic phosphorous compounds such as dichloroarylphosphines, chlorodiarylphosphines and arylphosphonic dichlorides, as compounds which extend the pot life of the molding sand composition. Further, U.S. Pat. No. 4,540,724 (Patent Document 2) proposes phosphorus halides such as phosphorus oxychloride, phosphorus trichloride and phosphorus pentachloride. Also, isophthalic acid chlorides and acids such as salicylic acid, benzoic acid, phosphoric acid, acidic phosphoric ester and boric acid have been conventionally used as the pot life extenders, because they inhibit the urethanization reaction.
Although the above-described conventional pot life extenders have an effect of inhibiting the urethanization reaction to retard curing of the molding sand composition, a further effect cannot be achieved even where they are added in an excessive amount over a predetermined amount, so that a sufficiently long pot life has not been achieved by addition of these conventional pot life extenders. Further, such pot life extenders inhibit catalysis of amine-based curing catalysts used for promoting the above-mentioned urethanization reaction, giving rise to inherent problems of uneven curing and reduction of the mold strength, which would result in formation of a hole in the mold at the time of its removal from a forming mold, and a further inherent problem that the amine-based curing catalysts are required to be used in larger amounts than necessary, in order to prevent such uneven curing and reduction of the mold strength.